Adverse cutaneous photosensitivity reactions following exposure to environmental chemicals and light have been reported with increasing frequency during recent years. The broad objectives of the proposed project are to define more precisely the mechanisms of action of these adverse photoresponses as well as the influence of environmental factors and the biologic differences between individuals which affect these reactions. Our approach to these objectives places particular emphasis on relatively unexplored aspects of this problem. Specifically, we are concerned with the role of temperature and humidity as they relate to photosensitivity reactions and the role of biologic factors of the host resulting in endogenously produced photosensitizers such as porphyrins. Although exogenous photosensitizing substances are well recognized causes of occupational dermatoses due to their primary irritant ("phototoxic") or allergic ("photoallergic") properties, little is known regarding the mechanisms of their photosensitizing properties on a molecular and submolecular biologic level. In addition, there presently exists no laboratory model for the study of the disabling photosensitivity in man known as persistent-light-reactors. The development of an experimental laboratory animal model for study of these reactions is a highly desirable goal in order to determine the specific role of light in the production of photosensitivity by "simple" chemical compounds. The value of these experimental models is to obtain knowledge of mechanisms of action which will aid in development and assessment of photoprotective topical and systemic agents. In addition to the use of in vitro models and in vivo animal experimentation, our studies will deal with the recognition of new adverse photosensitivity reactions in man due to various compounds which are being introduced into our environment by new developments in industry, manufacturing, and medical practice.